For example, as described in U.S. Pat. No. 5,505,440 (Apr. 9, 1996), as an apparatus for processing sheets, a mail processing apparatus for taking out mails one by one and reading information from them, postmarking the position of each postage stamp, and then stacking them respectively on stackers corresponding to reading results is known. This apparatus has a shingler conveyor for positively shifting mails taken out in an overlapped state and detecting overlapping. The mails detected overlapping by the shingler conveyor are rejected without being processed.
The shingler conveyor has a pair of belts for holding mails and moving in the same direction at different speeds and also has an upper stream side sensor for detecting the length of each of mails sent to the shingler conveyor in the conveying direction and a lower stream side sensor for detecting the length of each of mails sent out from the shingler conveyor in the conveying direction. And, the shingler conveyor compares the lengths of the mails measured by the two sensors, judges that when the lengths are different, overlapped sheets are mutually shifted, and detects overlapping.
However, for example, although two mails in the overlapped state are shifted, they cannot be shifted so that the lengths of the mails in the conveying direction are varied and when the shingler conveyor cannot detect overlapping of the sheets, to the canceling unit arranged on the lower stream side in the conveying direction, the mails are sent in the overlapping state. In this case, when canceling the stamps with a postmark, a problem arises that the canceling unit cancels the stamp of the mail only on the canceling hub with a postmark or the canceling position is shifted.